i had similar experience when i was young... had this girlfriend who i was really nice to.. i spend all my time for her... buy her everything which i cannot afford... but then she runs off with a married man..
sigh... i was young well educated.. from a good family... i don't look too bad... but she chooses this guy who is 15 years older than her instead...
but now i don't think about these things... just let things go naturally .. if she doesnt appreciate you then move on... there will be others for you...
Also, if you are really upset, its probably because you GAVE too much... take it a step at a time and don't put some much of your love in until you are both at the same level...